An X-ray image intensifier has a vacuum envelope consisting of glass, and an input window consisting of aluminum and having a sectional meridian radius of curvature which increases from the central portion of the input window to the peripheral portion thereof is arranged on the input side of the vacuum envelope with a metal holding ring and a Kovar ring. An input phosphor surface is arranged adjacent to the inner surface side of the input window, and an X-ray image incident through the input window is converted into a photoelectron image. In order to minimize an influence caused by scattering of X-rays or .gamma.-rays incident through the input window, an input substrate is brought as close to the input window as possible. A coaxial cylindrical focusing electrode and an annular focusing electrode are arranged on the side wall in the vacuum envelope, and an anode is arranged on an output end side. An output window is formed on the output side of the anode, and an output phosphor member is arranged on the inner surface side of the output window. A transmittance with respect to a radiation beam increases in inverse proportion to the energy of the radiation beam. A transmittance abruptly increases in an area extending from the intermediate portion of the input window to the outermost periphery of the input window.